Subgroup E avian type C viruses produced by bromodeoxyuridine-treated 100 X 7, line 7, or line C chicken cells were restricted in their intracellular growth on K28 chicken cells but not on line 15 chicken cells. Cells from embryos of line 15 chickens bred with K28 chickens did not restrict the growth of the subgroup E induced leukosis viruses (ILVs). This result indicates that the phenotype for the intracellular restriction of the growth of subgroup E ILVs found in K28 cells is recessive. Long-term growth of the subgroup E ILVs in K28 cells resulted in the appearance of subgroup E virus that grew well on K28 cells. No change in growth characteristics was observed for subgroup E ILVs grown in line 15 cells indicating that appearance of nonrestricted virus occurred only during growth of the subgrouo E ILVs on a restrictive host. RAV-0, a subgroup E virus closely related to the ilvs, had the same growth characteristics as the subgroup E ILVs. RAV-60, a subgroup E virus formed by recombination of exogenous avian leukosis virus with endogenous subgroup E virus coat information, grew well on both line 15 and K28 cells.

Infections with RNA viruses are sensed by the innate immune system through membrane-bound Toll-like receptors or the cytoplasmic RNA helicases RIG-I and MDA-5. It is believed that MDA-5 is crucial for sensing infections by picornaviruses, but there have been no studies on the role of this protein during infection with poliovirus, the prototypic picornavirus. Beginning at 4 h postinfection, MDA-5 protein is degraded in poliovirus-infected cells. Levels of MDA-5 declined beginning at 6 h after infection with rhinovirus type 1a or encephalomyocarditis virus, but the protein was stable in cells infected with rhinovirus type 16 or echovirus type 1. Cleavage of MDA-5 is not carried out by either poliovirus proteinase 2Apro or 3Cpro. Instead, degradation of MDA-5 in poliovirus-infected cells occurs in a proteasome- and caspase-dependent manner. Degradation of MDA-5 during poliovirus infection correlates with cleavage of poly(ADP) ribose polymerase (PARP), a hallmark of apoptosis. Induction of apoptosis by puromycin leads to cleavage of both PARP and MDA-5. The MDA-5 cleavage product observed in cells treated with puromycin is ~90 kDa, similar in size to the putative cleavage product observed in poliovirus-infected cells. Poliovirus-induced cleavage of MDA-5 may be a mechanism to antagonize production of type I interferon in response to viral infection.

Superinfection exclusion is the ability of an established virus infection to interfere with infection by a second virus. In this study, we found that Huh-7.5 cells acutely infected with hepatitis C virus (HCV) genotype 2a (chimeric strain J6/JFH) and cells harboring HCV genotype 1a, 1b, or 2a full-length or subgenomic replicons were resistant to infection with cell culture-produced HCV (HCVcc). Replicon-containing cells became permissive for HCVcc infection after treatment with an HCV-specific protease inhibitor. With the exception of cells harboring a J6/JFH-FLneo replicon, infected or replicon-containing cells were permissive for HCV pseudoparticle (HCVpp) entry, demonstrating a postentry superinfection block downstream of primary translation. The surprising resistance of J6/JFH-FLneo replicon-containing cells to HCVpp infection suggested a defect in virus entry. This block was due to reduced expression of the HCV coreceptor CD81. Further analyses indicated that J6/JFH may be toxic for cells expressing high levels of CD81, thus selecting for a CD81low population. CD81 down regulation was not observed in acutely infected cells, suggesting that this may not be a general mechanism of HCV superinfection exclusion. Thus, HCV establishes superinfection exclusion at a postentry step, and this effect is reversible by treatment of infected cells with antiviral compounds.